Method of producing steel products having excellent internal quality

ABSTRACT

A method of producing a steel product having an excellent internal quality including subjecting a steel raw material of a round section to rolling at 3 or more passes to form a steel product of round section, wherein the rolling is conducted with a pair of upper and lower flat rolls at first pass, a pair of upper and lower same or different caliber rolls at second or more passes until just before a last pass, and a pair of upper and lower round caliber rolls at the last pass, under a condition that an area reduction in the first pass is within a range of less than a total area reduction from the raw material to the product.

TECHNICAL FIELD

This disclosure relates to a method of producing steel products havingexcellent internal quality.

BACKGROUND

In general, when a steel raw material with a round section (which isalso called as a circular section) is rolled, the resulting product hasalso a round section. In that case, a series of caliber rolls with oval(ellipsoid; brevity code O)-round (circle; brevity code R) arefrequently used. On the contrary, when the shape of the raw material isrectangular, a sectional area (cross sectional area for details, thesame is used hereafter) is reduced by caliber rolls with a groove shapeof square (square; brevity code S), box (hexagon; brevity codeB)-diamond (rhombus; brevity code D) or the like to finally provide adesired shape. Of course, when the raw material has a square section, acombination of square (S)-oval (O) or the like is also used (Journal ofNippon Plastic Working Associate, Vol. 24-273 (1983. 10) p. 1070-1077).FIG. 4 shows various examples of the above shapers for the caliber rolltogether with a shape of a flat roll (flat roll; brevity code F).Moreover, FIG. 4 shows a sectional view of upper and lower rolls cut bya plain surface passing through their shaft center lines. In FIG. 4, F/Fis an abbreviation of upper and lower flat rolls, O/O is an abbreviationof upper and lower oval caliber rolls, R/R is an abbreviation of upperand lower round caliber rolls, S/S is an abbreviation of upper and lowersquare caliber rolls, B/B is an abbreviation of upper and lower boxcaliber rolls, and D/D is an abbreviation of upper and lower diamondcaliber rolls (the same is used hereafter).

Especially, when the shape of the raw material or an intermediatematerial is approximately circular, a final shape of a circular sectionis manufactured by oval (O) or round (R) rolls as mentioned above.

On the other hand, when the raw material is an as-cast steel billet orretains defects in a sectional center portion of the steel billet, it isunsuitable as a final product or a raw material directing to anotherproduction line. Because the retained defects lead to flaws throughfurther working or begin at the occurrence of breakage or the like insubsequent rolling. When a product is manufactured from the steel billetsuch as a slab or the like, a strong drafting way is known as a methodof solving the defects in the central center of the raw material byrolling (The Iron and Steel, '81-S339). Thus, when the drafting of, forexample, 30 mm is necessary in the production of the steel sheet,products having an excellent internal quality are obtained by conductingthe drafting of 30 mm at once rather than three times of rolling of 10mm/pass.

Moreover, Japanese Patent No. 3649054 discloses that as a rolling methodto prevent rolling cracks in the continuously cast steel billet(particularly rolling cracks of the side face), when a steel bar ismanufactured from the continuously cast steel billet by direct rolling,a continuously cast steel billet of a round section is used and acaliber roll is used in a first pass of rough rolling and a flat roll isused in second pass or more of the rough rolling.

However, when the raw material has a round section and, further, theproduct has also a round section, it is not necessarily easy to applythe technique capable of simply repressing the strong drafting. Because,an area reduction ratio of decreasing a cross sectional area of a steelbillet or cast slab as a raw material to a cross sectional area of afinal product (shortly referred to as area reduction; =1−sectional areaof product/sectional area of raw material), i.e. rolling reduction ispreviously decided and also a groove shape or rolling reduction requiredin shaping is limited to a certain extent. Alternatively, there is amethod of increasing the sectional area of the raw material. In eachcase, however, much time and labor are used to change the groove shapeor optimize the rolling reduction, which is industrially difficult. Inaddition, when the raw material is produced from a mold, there is alarge restriction and is difficult as a practical matter. As previouslymentioned, a groove shape of approximately an ellipsoid is usually whenrolling the round section, but when the strong drafting is conductedwith such a groove shape, protruding from the groove shape andover-filling is caused, which results in fear of retaining flaws on thesurface of the product. If the drafting is deficient, a portion notfilled in the groove shape retains on the surface of the final productwithout over-filling so that it is difficult to apply the strongdrafting approach. Hence, the internal quality and shape may not besufficiently satisfied, which becomes a problem.

SUMMARY

We examined defects existing in the round sectional center of the rawmaterial and effectively blocked them even if the rolling reduction isnot necessarily high in the hot rolling of 3 or more passes usuallyadapting the caliber rolling to provide a desired product shape. Wefound that the strong drafting approach can be easily applied when therolling in only first pass of the hot rolling is conducted with upperand lower flat rolls and second or more passes are conducted by caliberrolling to thereby obtain a product having sufficient internal qualityand shape.

We thus provide a method of producing steel products having an excellentinternal quality by subjecting a steel raw material of a round sectionto rolling of 3 or more passes to provide a steel product of a roundsection, characterized in that the rolling is conducted by using a pairof upper and lower flat rolls at first pass, using a pair of upper andlower same or different caliber rolls at second or more passes untiljust before a last pass, and using a pair of upper and lower roundcaliber rolls at the last pass, under a condition that an area reductionin the first pass is within a range of less than a total area reductionfrom the raw material to the product. It is preferable that the areareduction in the first pass is not less than 50% of a total areareduction in the second or more passes.

Defects existing in the center of the round section can be sufficientlyblocked by strong draft rolling with the upper and lower flat rolls atthe first pass, while the section flattened by the strong drafting canbe sufficiently circles by the caliber rolling with a relatively lightdrafting at the second or more passes, whereby a steel product of roundsection having a satisfactory internal quality is obtained withoutdeteriorating the shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the influence of a roll shape upon aninterrelation between defect blocking ratio and area reduction in onepass rolling (results of Experiment 1).

FIG. 2 is a graph showing the influence of a roll shape upon aninterrelation between defect blocking ratio and area reduction in onepass rolling (results of Experiment 2).

FIG. 3 is a graph showing the influence of a roll shape upon aninterrelation between defect blocking ratio and area reduction in onepass rolling (results of Experiment 3).

FIG. 4 is a schematic view illustrating various shapes of caliber rollsand a shape of as flat roll.

DETAILED DESCRIPTION

We conducted experiments on how to change the interrelation between adefect blocking ratio (=1−sectional area of defect afterrolling/sectional area of defect in raw material) and the area reduction(=1−sectional area of rolled product/sectional area (including sectionalarea of defect) of raw material) in accordance with a shape of a rollused when a raw material of a round section provided with an artificialdefect passing through a central portion of the round section is rolledat one pass. In this experiment, a lead raw material was used to conductcold rolling. This can be adopted as a good approach because colddeformation behavior of lead is close to hot deformation behavior(1000˜1200° C.) of steel and also deformation resistance of lead at roomtemperature tends to be substantially equal to hot deformationresistance of steel.

Experiment 1

In Experiment 1, the raw material has an outer diameter=50 mmφ and adefect diameter=5 mmφ, and shapes of upper and lower rolls are fourkinds of F/F, D/D, O/O and B/B (see FIG. 4), and the roll diameter is 5times of the diameter of the raw material (wherein a diameter of a rollflange portion is used for a roll diameter of a caliber roll), and thearea reduction is varied within a range of not more than about 25%. Thesectional area of the defect after the rolling is determined from animage of the defect shot in a section of a rolled product. Moreover,when the defect is not observed from the shot image, a color check testis carried out on the section to be shot to confirm no transudation of apenetrating solution (the same is used hereafter).

The results are shown in FIG. 1. As seen from FIG. 1, the defectblocking ratio increases together with the area reduction even in anyroll shape, but the increasing tendency is particularly steep in thecase of F/F as compared to the other cases, and also complete blocking(defect blocking ratio=1) is attained at an area reduction=about 21%,which indicates that the defect blocking ratio can be largely enhancedby the strong drafting with the upper and lower flat rolls.

Experiment 2

Experiment 2 used the same specifications as Experiment 1 except thatthe defect diameter is 2.5 mmφ. The results are shown in FIG. 2. As seenfrom FIG. 2, the defect blocking ratio increases together with the areareduction even in any roll shape, but the increasing tendency isparticularly steep in the case of F/F as compared to the other cases,and also complete blocking (defect blocking ratio=1) is attained at anarea reduction=about 21%, which indicates that the defect blocking ratiocan be largely enhanced by the strong drafting with the upper and lowerflat rolls.

Experiment 3

Experiment 3 used the same specifications as Experiment 1 except thatthe outer diameter of the raw material is 30 mmφ and the defect diameteris 3 mmφ. The results are shown in FIG. 3. As seen from FIG. 3, thedefect blocking ratio increases together with the area reduction even inany roll shape, but the increasing tendency is particularly steep in thecase of F/F as compared with the other cases, and also complete blocking(defect blocking ratio=1) is attained at an area reduction=about 9%,which indicates that the defect blocking ratio can be largely enhancedby the strong drafting with the upper and lower flat rolls.

Next, a pass applying F/F (upper and lower flat rolls) rolling isexamined among 3 or more rolling passes, and hence the followingconclusion is obtained. Since the strong draft is conducted in the F/Frolling, when the strong draft is carried out at second or more passes,if there is a limit in the pass number, the number of caliber rollingpasses from the pass after the strong draft to final pass is decreased.Hence, it is difficult to render the final section into a true circle.If there is no limit in the pass number, formation of the true circlemay be made possible by further adding caliber rolling stands, but thenumber of the stands is increased, which is a large demerit in therolling efficiency and for economical reasons. Therefore, the F/Frolling should be carried out only at the first pass.

The area reduction in the F/F rolling (first pass) should be less than agiven total area reduction from the raw material to the product. Ingeneral, a total area reduction from an entry side of mth pass to anexit side of nth pass (m<n) (represented by symbol of Z_(m/n)) isdefined by an equation (1) from sectional area at the entry side of mthpass S_(m-1) and sectional area at the exit side of nth pass S_(n):

Z _(m/n)=1−S _(n) /S _(m-1)  (1)

When total area reduction is a range from the raw material (entry sideof first pass) to the product (exit side of final Nth pass), theequation (1) is changed into an equation (2) since m=1 and n=N.

Z _(1/N)=1−S _(N) /S ₀  (2)

The equation (2) is deformed to an equation (3) by using sectional areaSi and area reduction z_(i) (=1−S_(i)/S_(i-1)) at an exit side of ithpass:

Z _(1/N)=1−(1−_(i)Π_(1/N)(1−z _(i))  (3)

wherein _(i)Π_(1/N)(1−z_(i))≡(1−z₁)(1−z₂) . . . (1−z_(N)).

Since each of the sectional area S₀ of the raw material and targetsectional area S_(N) of the product is a given value, the total areareduction Z_(1/N) from the raw material to the product is also a givenvalue. When z₁≧Z_(1/N), 1−_(i)Π_(2/N)(1−z_(i))=Z_(2/N)≦0 from theequation (3), so that the caliber rolling at second or more passescannot be conducted and hence the target shape of round section is notobtained. Therefore, there should be z₁<Z_(1/N).

On the other hand, when z₁ is less than 50% of Z_(2/N), the strong draftis not obtained so that there is a possibility that the defect blockingeffect is poor. Since we believe that when the defect blocking iscarried out at the first pass, only the arrangement of the shape issufficient at the remaining passes, the area reduction z₁ of the F/Frolling (first pass) is preferable to be not less than 50% of the totalarea reduction Z_(2/N) of the caliber rolling.

Example

A through-hole (circular section) is pierced in a sectional center of asteel raw material of a round section as an artificial defect to form atest specimen, which is heated and hot rolled under various rollingconditions to provide a steel product having a target round section.Then, there are examined right and wrong in the defect blocking ratioand shape of the resulting steel product. Table 1 shows dimensions(outer diameter, defect diameter) of the raw material used, target size(outer diameter) of the steel product, total area reduction Z_(1/N) androlling conditions (total pass number N, shape of roll used (F/F→O/O . .. →R/R and so on), area reduction at first pass z₁, total area reductionof second or more passes Z_(2/N)) from entry side of first pass to exitside of final Nth pass. Moreover, the heating temperature is 1100° C.The roll diameter of the flat roll is 200 mm, and the roll diameter ofthe caliber roll (roll diameter at flange end) is 200 mm. Thetemperature at exit side of the final pass is lowered to about 50-100°C. from the heating temperature.

The defect blocking ratio of the resulting steel product is examined bythe same manner as in the above experiments. As the right and wrong ofthe shape, a ratio of minimum diameter/maximum diameter incircumferential direction is measured as an indication of true circle,and the shape is judged to be good (◯) when the indication of truecircle is not less than 0.975 and bad (x) other than that. These resultsare shown in Table 1.

As seen from Table 1, the defect is completely blocked and the shape isgood in our Examples (F/F only at first pass, and z₁<Z_(1/N)).

TABLE 1 Raw material Target outer Outer Defect diameter of Defectdiameter diameter steel product Z1/N Z1 Z2/N blocking No. (mm) (mm) (mm)(%) N Shape of rolls used (%) (%) ratio Shape Remarks 1 50 5 37.0 45 4F/F→O/O→O/O→R/R 25 27 1 ∘ Example 2 50 3 33.5 55 4 F/F→D/D→O/O→R/R 25 401 ∘ Example 3 50 3 33.5 55 4 F/F→O/O→O/O→R/R 33 33 1 ∘ Example 4 30 322.3 45 4 F/F→O/O→O/O→R/R 15 35 1 ∘ Example 5 30 3 24.0 36 4F/F→O/O→O/O→R/R 15 25 1 ∘ Example 6 30 3 25.5 27 3 F/F→O/O→R/R 15 14 1 ∘Example 7 50 5 32.5 58 4 F/F→D/D→O/O→R/R 40 30 1 x Comparative Example 850 3 37.0 45 4 F/F→D/D→O/O→R/R 15 35 0.81 ∘ Comparative Example 9 30 322.3 45 4 F/F→D/D→O/O→R/R 8 40 0.7 ∘ Comparative Example 10 50 5 40.0 363 F/F→O/O→R/R 25 15 1 x Comparative Example 11 50 5 37.0 45 4F/F→O/O→O/O→R/R 16 35 0.85 ∘ Comparative Example 12 50 5 40.0 37 3F/F→O/O→R/R 13 28 0.68 x Comparative Example

1-2. (canceled)
 3. A method of producing a steel product having anexcellent internal quality comprising subjecting a steel raw material ofa round section to rolling at 3 or more passes to form a steel productof round section, wherein the rolling is conducted with a pair of upperand lower flat rolls at first pass, a pair of upper and lower same ordifferent caliber rolls at second or more passes until just before alast pass, and a pair of upper and lower round caliber rolls at the lastpass, under a condition that an area reduction in the first pass iswithin a range of less than a total area reduction from the raw materialto the product.
 4. The method according to claim 3, wherein the areareduction in the first pass is not less than 50% of a total areareduction in the second or more passes.